Line scanning in a display

ABSTRACT

The present invention is directed towards a method of scanning lines in a display and towards a display control device for scanning lines in a way which is less trackable by the human eye as well as towards an electronic device including such a display control device. The control device includes a line driving unit ( 18,20 ) and a control unit ( 22 ), which control unit ( 22 ) varies the selection of lines to be scanned and controls the line driving unit ( 18,20 ) to scan the selected lines. Selection and scanning is varied in such a way that energy variations caused by the scanning of lines is less trackable by the human eye. In this way bright lines or flickering normally caused by scanning of lines is reduced.

TECHNICAL FIELD

The present invention is directed towards a method of scanning lines ina display, a display control device as well as an electronic deviceincluding such a display control device and more generally towardsselection of rows or columns for scanning in displays.

BACKGROUND OF THE INVENTION

In some types of displays, like in passive addressed organic LED matrixdisplays, when a row is to be illuminated during a flame, all the lightis generated during the time the row is active or scanned. In a smallamount of time a large peak brightness is then generated. If the displaycontains for example 100 rows, this leads to a row having 100 timesgreater brightness than the average brightness of the display.

In this type of display rows are normally scanned by consecutive rowscanning, i.e. scanning all rows of the display, starting with the firstand then sequentially scanning all the other rows ending with the lastrow of the display. This scanning is done within a time called a frameand scanning of the rows is repeated for each frame.

It is also known to use what is called interlaced scanning. Here, firstall the even rows are scanned in growing order followed by scanning ofall the odd lines in growing order from a first row to a last row of thedisplay. This means that every other line is scanned during a first halfof a frame and then the rest of the lines are scanned during the secondhalf of the frame.

If a consecutive or interlaced row scanning order is used in a passiveaddressed organic LED matrix display, then rows are energized with aconstant scanning speed. If a viewer watches the display from the cornerof his eyes without focusing on the screen or the display is moved at aconstant speed in the opposite direction to the scanning direction or ifthe human eye moves in the same direction as the scanning directionwhile viewing the display and this movement has the same speed as thescanning speed, a bright flashy line will appear to the eye, which isdistressing to the observer. This problem arises because the light ofactivated pixels in consecutive lines stimulates the same nerves withinthe retina of the eye. This is also known as saccadic eye movement. Theproblem can also occur when a user is blinking. This problem can also bepresent in other types of displays, but is often most prominent inpassive addressed organic LED matrix displays.

If the same type of line scanning is continually used for a number ofconsecutive frames another problem might arise for specific anglevelocities of the eye. Possibly, no emissive pixel is observed at allduring a number of frames, resulting in a perceived modulation of theluminance level between the average level and black. Especially whenlarge parts of the picture are at the same luminance level, temporarilydarker parts are observed when looking for a long period. The perceptionof this adding up of so-called black “off-pixels” differs from theperception of the adding up of bright “on-pixels”. These added up black“off-pixels” result in the perception of flicker by the eye.

U.S. Pat. No. 5,796,375 describes driving a display with different timeperiods for providing a certain luminance level. Here rows are scannedin consecutive order. The document also describes the division of thescreen into a number of tiles, each comprising sixteen rows. The rowswithin a tile are scanned in successive order. In this way the differenttiles or every sixteenth row of the display are scanned simultaneously,but in successive order. The document also describes reducing theflicker level of the display. Luminance levels for a pixel are normallyprovided during eight fields of data having different time periods in aframe, where the lowest intensity is provided in one time period and thehighest in all eight. By displaying these fields in a non-progressivesequence, for instance in a random order, the flicker of the display isreduced. This reduction of a flicker level is thus related to theluminance level of a pixel and does not reduce the above-mentionedproblems related to the scanning of a whole line of pixels such as thescanning of a row.

SUMMARY OF THE INVENTION

The present invention is directed towards solving or at least reducingthe above-mentioned problems associated with the piling up of black“off-pixels” causing flickering or the piling up of bright “on-pixels”causing bright lines at saccadic eye-movement, i.e. when the display iswatched from the corner of the eye without focusing, or when the displayis moved or the eye is moved during scanning of lines in the display.The invention is defined by the independent claims. The dependent claimsdefine advantageous embodiments.

The problems are solved by a method of scanning lines in a displayincluding the steps of selecting lines to be scanned, scanning the linesand varying selection and scanning of lines so as to reduce tracking bythe human eye of energy variations caused by scanning. The problems arealso solved by a display control device and an electronic deviceincluding such a display control device, the display control devicecomprising a line driving unit and a control unit. The control unit isarranged to vary selection of lines to be scanned and to control theline driving unit to scan the selected lines so as to reduce tracking bythe human eye of energy variations caused by scanning. The display canbe an organic LED display, as well as any other type of display whichapplies scanning in combination with a high peak-intensity ratio ofpixels in relation to the average intensity to be displayed, such as aField Emission Display (FED).

It is advantageous if the method further comprises the steps ofselecting a line to be scanned during a frame of the display accordingto a non-consecutive selection criterion, scanning the selected line,continuing selecting other lines according to the non-consecutiveselection criterion and scanning the other lines until at least a set oflines of the display have been scanned during the frame, wherein thenon-consecutive selection criterion provides at least two different stepsizes to be used when selecting lines within the set.

This first embodiment of the present invention is directed towardssolving the problem with bright lines that appear during saccadic eyemovement, i.e. when a display is watched from the corner of the eyewithout focusing, during scanning or energizing of a whole line ofpixels or when the display is moved or the eye is moved during scanning.

It is also advantageous if the method further comprises the steps ofselecting a first selection criterion for a first frame, using saidcriterion for the first frame, selecting a second selection criterionfor a second frame and using said second criterion for the second frameso that line selection and scanning is varied between frames.

The second embodiment is directed towards solving the problem of piledup black off-pixels, which makes the eye perceive flickering at saccadiceye movement, i.e. when the display is watched from the corner of theeye without focusing, or which appears when the display is moved or theeye is moved during scanning or energizing of a whole line of pixels.

The problems are also solved by a display control device comprising aline driving unit and a control unit, wherein the control unit isarranged to vary selection of lines to be scanned and to control theline driving unit to scan the selected lines so as to reduce tracking bythe human eye of energy variations caused by scanning. The controldevice can be an electronic unit, circuitry or one or more integratedcircuits with or without peripheral components.

It is advantageous if the control unit is arranged to choose a firstnon-consecutive selection criterion, use said first criterion during atleast a first frame, choose a second non-consecutive selection criterionand use said second criterion during at least a second frame so thatline selection and scanning is varied between frames. This embodiment isdirected towards solving the same problem as the second embodiment.

The idea of the invention is to provide line scanning which is nottrackable by the human eye, so that irritating bright lines or otherflickering normally caused by line scanning is avoided.

The expression line used here is intended to comprise lines in anydirection on the display, i.e. either in row or column direction.

The above mentioned and other aspects of the invention will be apparentfrom and elucidated with reference to the embodiments describedhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be further described in relation to theaccompanying drawings, in which:

FIG. 1 shows an electronic device according to the invention,

FIG. 2 shows a block schematic of a display control device according tothe invention,

FIG. 3 shows a first division of a display into a set of rows forexplaining the principle of a first embodiment of the invention,

FIG. 4 shows a second division of a display into different sets of rowsfor explaining the principle of an alternative embodiment of the firstembodiment of the invention,

FIG. 5 shows a flow chart of a method according to the first embodiment.

FIG. 6 shows a side view of an LCD layer and a backlighting layer of adisplay,

FIG. 7 shows a block schematic of a display control device forbacklighting a display, and

FIG. 8 shows a flow chart of a method according a second embodiment ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 is shown an electronic consumer device 10 in the form of acellular phone provided with a display 12, which preferably is a passiveaddressed organic LED matrix display. The organic LED display can be asmall molecule or polymer LED matrix display. A user looks at thescanned line of a display with an eye 14. When the user looks at thescreen of the device 10 with saccadic eye movement, i.e. from the cornerof the eye without focusing, or when he blinks or watches the displaywhile either moving the display or the eye with constant speed, he willnot be able to track a flashing line when applying the scanning schemeaccording to the invention as will be elucidated below. In the Figure,the user moves his eye in the normal scanning direction, which isindicated by means of an arrow.

FIG. 2 shows a block schematic of a display control device according tothe invention. The display control device includes an image memory 16containing data about how each pixel of a display is to be displayed.The content of the image memory is decided by such things as functionsactivated by the user and different types of input from a user as wellas a general layout of the screen, and is well known within the art.This information can include such things as color and brightness. Thedevice furthermore includes a first line driving unit in the form of acolumn driving unit 20 connected between the image memory 16 and thedisplay 12. The device also includes a second line driving unit in theform of a row driving unit 18 connected to the display 12. There isfurthermore a timing and control unit 22 connected to the image memory16, the column driving unit 20 and the row driving unit 18. The timingand control unit is preferably provided in the form of a processor withassociated program memory containing program code for providing the rowselection scheme. As mentioned before, the image memory 16 includes dataabout how all pixels of the display are to be displayed. The imagememory receives this information from other parts of the phone in amanner well known within the art, for instance from the receiving partof a mobile phone, television receiver, or the information is inputtedin correct form from any input device. All pixels are energized once perframe. This is done through the timing and control unit 22 controllingthe image memory 16 to submit data about each row of pixels to thecolumn driving unit 20 and then provide the row driving unit 18 withsignals selecting rows for scanning. When a row is selected the columndriving unit 20 applies a pixel pattern received from the image memory16 to the columns and the row is scanned or energized so that the pixelsare lit. How this is done is largely well known in the art and will notbe further described here. What is not well known however is the way inwhich rows are selected, which is the subject of a first embodiment ofthe present invention.

The first embodiment of the invention will now be described in relationto FIG. 3, which shows the rows of the display 12. The display typicallyhas 2N rows, where N can be for instance 50. The display can thereforebe seen as containing a set of rows consecutively numbered from 1 to 2N.According to one of the known scanning techniques, the rows would bescanned in sequence from row no. 1 to row no. 2N, without skipping arow. The distance between consecutively selected rows would then beconstant. According to the first embodiment another way of row selectionis used. First a row in the middle of the display, row no. N, is chosenand then scanned. Thereafter rows N−1 and N+1 are selected and scannedby subtracting the scanned row in the downward direction by one andadding the scanned row in the upward direction by one. Thereafter rowsare scanned with alternating growing and diminishing orders until allrows have been scanned within a frame. In this way a non-consecutivescanning scheme has been provided, i.e. rows are not selected in aconsecutive order for scanning. The distance between sequentiallyselected rows also differs, i.e. the distance between the rows selectedafter each other changes. This can also be seen as a way to vary thescanning step size between sequentially selected rows. In this wayselection and scanning of rows has been varied such that the scanning ofrows is not trackable by the human eye. There is also a constantlychanging scanning direction, which makes that the eye is not able totrack the flashes. The user is thus unable to notice the flashesemerging because of the scanning of a line when the eye is moved or thedisplay is moved at saccadic eye movement. In this example the number offlashes that add up in the eye during constant movement at the criticalspeed is halved.

A method for scanning rows according to the first embodiment, which isalso considered to be a preferred mode to practice the invention, willnow be explained in relation to FIG. 5. When a set of 2n rows has to bescanned, the method involves the following steps:

-   In step 30 a first row counter RC1 is set to a value N,-   In step 32 the row N is scanned.-   In step 34 a second row counter RC2 is set to the value of the first    row counter RC1, so to the value N.-   In step 36 the value of the second row counter RC2 is set to its    previous value −1.-   In step 38 the row corresponding to the value of the second row    counter RC2 is scanned.-   In step 40 the first row counter RC1 is set to its previous value    +1.-   In step 42 the row corresponding to the value of the first row    counter RC1 is scanned.-   In step 44 is checked whether all rows of the set have been scanned:-   If not, indicated by N, then step 36 up to and including 44 are    repeated.-   If yes, indicated by Y, the method is ended, indicated by END, in    step 45.

The method can then be repeated for all consecutive frames starting withstep 30. In the above-mentioned scheme, there is a non-consecutivescanning scheme, i.e. the rows selected for scanning are not consecutiverows. Step sizes between the consecutively or sequentially selected rowsthus differ. In this way selection and scanning of rows has been variedsuch that the scanning of rows is not trackable by the human eye. Thereare thus different distances or step sizes between sequentially selectedrows, i.e. between the rows selected after each other, and alsodifferent scanning directions, i.e. scanning is alternately performed inan upward and a downward direction.

With the described embodiment, there is a chance that two successiverows coincide with the constant movement of the eye or display asdescribed above. However the piled up brightness is halved, whichreduces the problem.

Thus a preferred embodiment of the invention has been described.However, there are many ways in which the invention can be varied. Onefirst variation of the first embodiment of the present invention willnow be described in relation to FIG. 4. In this so called split screendisplay the display is divided into two sets of rows, where the firstset starts with row 1 and ends with row N and the second set starts withrow N+1 and ends with row 2N. The above-described method of selectingrows is performed for each set of rows, either serially or in parallel.This would mean that row N/2 would be scanned first in the first setfollowed by alternating scanning of lower and higher order rows untilrows 1 and N have been scanned. In the same way row 3N/2 would bescanned first in the second set followed by alternating scanning oflower and higher order rows until rows N+1 and 2N have been scanned inthe second set. Serial selection is to taken to mean in the method thatfirst the first set of rows is scanned followed by scanning of thesecond set of rows. Parallel scanning is to be taken to mean that thescanning of rows in the second set is done simultaneously with that ofrows in the first set. Simultaneous scanning can also be done in such away that when a row is scanned in the direction towards lower ordernumbers in the first set a row is scanned in the direction of higherorder numbers in the second set at the same time. All rows are scannedhowever within the same frame. It is possible to scan with moredifferent sets of rows arranged in parallel.

There are several more ways in which the first embodiment of the presentinvention can be varied. It is possible to use a completely random rowselection. Here the timing and control unit would include a randomnumber generator for selecting non-scanned rows for scanning. In thiscase there might exist the same step size between a few selected rows,but there will be at least two different step sizes used betweendifferent rows.

It is furthermore possible to use other types of scanning orders. Therecan be a scanning order where the distance between the rows whenscanning in a direction is doubled. For instance, scanning could be doneof rows 1, 2, 4, 8, 16, 32, 64 etc. up to row 2N and then the same typeof scheme would be used for scanning in the backward direction. Thescheme is repeated for all non-scanned rows within a frame until all therows have been scanned. The scheme can of course also be used for two ormore sets of rows in the same way as described above, either foraddressing serially or in parallel. It is also possible to combine thetwo different directions, in that the upward and downward scanning canbe performed alternately in a manner similar to what was described inthe first embodiment. It is also possible to scan if the distancebetween scanned rows has another growth order, like for instancetripling of the distance between the scanned rows.

A further variation is to scan a first group of consecutive rows, forinstance, rows 1, 2 and 3 followed by scanning of a second group ofrows, for instance rows 7, 8 and 9 and by scanning of a third group ofrows, for instance 4, 5 and 6, etc. Here there is one first step sizebetween the rows within a group and another step size between the lastrow of a group and the first row of a following group. This means thatthe selection criterion provides at least two different step sizes forconsecutively selected rows. In one embodiment there are also varyingstep sizes between the different groups of rows.

Naturally each one of these three last-mentioned variations of the firstembodiment can be combined with a split screen scanning.

These are just a few of all types of scanning schemes that can be used.The important thing is that it is not possible to track the scannedrows, which might occur with simple display movements or simple eyemovements, or during saccadic eye movement. In this way there arevarying distances and varying step sizes between consecutively scannedrows, so that a user cannot see a line movement when watching thedisplay from the corner of the eye. There are also provided at least twodifferent step sizes for selecting rows to be scanned within a set ofrows for each selection criterion.

The problem described above is most prominent when the display isshowing non-moving pictures with many pixels lit. In a black-and-whitedisplay this would mean that many pixels were showing the color white.However, the effect is also visible for lower intensities, for instancewith a gray background instead or when other colors are displayed.

The present invention according to the first embodiment can also be usedfor varying the backlighting used for a display. FIG. 6 shows a sideview of a display in a cellular phone provided with an LCD layer 12provided on top of a transparent backlighting layer 46. In the LCD layerinformation is displayed according to the principles described earlier.The backlighting layer 46 is provided for lighting up the display forinstance when the environment in which the display is used is dark.

In FIG. 7 is shown a block schematic of a device for scanning lines forbacklighting a display. Here the backlighting layer 46 is divided into anumber of segments, such that each segment can be seen as forming a lineto be lit up by a lamp. In FIG. 7 there are six lamps 48, 50, 52, 54, 56and 58 in the form of light emitting diodes. It is evident that othertypes of lamps are possible instead. There is also a control unit 60connected to the lamps, which controls the scanning of the lines. Thiscontrol unit is a combined control and line driving unit.

If the display is to be backlit, the control unit 60 selects lines to bescanned according to a non-consecutive selection criterion, for instanceaccording to any of the ones described earlier. All the lines arescanned during a frame. In this way the lines are not scanned in aconsecutive order that can be followed by the eye.

In the embodiment described, the control unit for backlighting differsfrom the timing and control unit used for the LCD layer. It can howeverbe the same control unit. Naturally there can also be more or fewer thansix lamps. The principle can furthermore be used for front-lit displaysas well.

The first embodiment of the invention and the variations mentioned thusavoid the possibility that bright scanned lines are detected by the eye.

In a second embodiment of the present invention, different row scanningschemes are provided for different frames. During a first frame a firstscheme is used, for instance the described preferred scheme. During asecond frame another scheme is used, for instance the scheme describedabove with doubled distances between scanned rows. In this way switchingbetween several different scanning schemes can be performed fordifferent frames. Said switching can be performed for each frame; onescheme can be used for a number of frames followed by another scheme fora few other frames etc. The possible variations of this switching arecountless. In this case the timing and control unit keeps track of whento switch between schemes. This second embodiment will be described inmore detail in relation to FIG. 8, which shows a flow chart of themethod in question. The device used for performing this method is thedevice in FIG. 2.

First the timing and control unit selects a first row selectioncriterion to be used for a first number of frames X and a second rowselection criterion to be used for a second number of frames Y, step 62.The numbers are preferably equal, but this is not necessary. The numberscan furthermore differ considerably from one frame up to tens orhundreds of frames. Thereafter a first frame counter FC1 is set to theselected number of frames X for which the first criterion is to be usedand a second frame counter FC2 is set to the selected number of frames Yfor which the second criterion is to be used by the timing and controlunit, step 64. Thereafter the timing and control unit selects rowsaccording to the first criterion, makes the row driving unit scan theselected rows during a first one of the frames and decreases the firstframe counter FC1 by one, step 66. If the first frame counter FC1 hasnot reached zero, step 68, the selection and scanning according to thefirst criterion is continued, step 66. If FC1 has reached zero, step 68,the timing and control unit switches selection criterion to the secondrow selection criterion, selects rows according to this criterion, makesthe row driving unit scan the thus selected rows during a first frameand decreases the second frame counter, FC2 by one, step 70. If FC2 hasnot reached zero, step 72, the timing and control unit continuesselecting rows according to the second criterion and continues makingthe row driving unit scan these selected rows, step 70. If FC2 hasreached zero, step 72, the timing and control unit again sets FC1 andFC2 to the originally set values, step 64, and continues running throughsteps 66-72.

There are a number of possible variations of this second embodiment ofthe invention. The schemes selected are preferably the ones usedaccording to the first embodiment of the invention. This is however notnecessary. Other schemes are possible, like for instance interlaced andcontinuous row selection. It is furthermore possible to alternatebetween more than two different criteria. Finally it is also possible tochange selections of criteria and the number of frames during which acriterion is to be used after scanning has started.

It is furthermore possible to vary the second embodiment of theinvention in the following way. Different selection criteria can be useddepending on data content or mode of operation. For example onecriterion can be used when text is scrolled, another when graphics is tobe displayed and perhaps yet another criterion is used when the displayis in stand-by mode.

With this second embodiment and the variations mentioned, detection offlickering because of piled up black “off-pixels” is avoided.

The present invention is not limited to use in combination with passivematrix organic LED matrix displays, but can be used in any type ofdisplay with high peak brightness of the pixels. Examples of other typesof displays are PDP, OLED, EL, and CRT. The second embodiment andvariations thereof can furthermore be provided in combination with anActive Matrix Address LCD display with scanning backlight, where thenumber of black light sequences is increased.

The invention is furthermore not limited to electronic devices likecellular phones, but can be implemented in any type of electronic devicesuch as palmtops, laptop computers, electronic game machines, TVs andstandard computers. The electronic device can also be a display modulecomprising the display, the line driving unit and the control unit.Finally, it is equally well possible to use the same type of scanningfor columns instead of rows.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The invention can be implemented by means of hardware comprising severaldistinct elements, and by means of a suitably programmed computer. Inthe device claim enumerating several means, a number of these means canbe embodied by one and the same item of hardware. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate that a combination of these measures cannot be used toadvantage.

1. Method of scanning lines in a display, the method including the steps of selecting lines to be scanned, scanning the lines and varying selection and scanning of lines so as to reduce tracking by a human eye of energy variations caused by scanning.
 2. Method according to claim 1, further including: selecting a line to be scanned during a frame of the display according to a non-consecutive selection criterion, scanning the selected line, continuing selecting other lines according to the non-consecutive selection criterion, and scanning the other lines until at least a set of lines of the display have been scanned during the frame, wherein the non-consecutive selection criterion provides at least two different step sizes to be used when selecting lines within the set.
 3. Method according to claim 2, wherein the set of lines comprises a set of consecutive lines of the display less than all lines of the display.
 4. Method according to claim 2, wherein the set of lines includes all lines of the display.
 5. Method according to claim 2, wherein the non-consecutive selection criterion comprises selecting a line in between the first and the last line and thereafter alternately selecting a lower order line and a higher order line relative to the first selected line until all lines of the set of lines have been scanned.
 6. Method according to claim 2, comprising the step of changing the scanning direction between at least some sequentially selected lines within the set.
 7. Method according to claim 2, wherein the step sizes are varied between sequentially selected lines or groups of lines.
 8. Method according to claim 2, wherein the non-consecutive selection criterion comprises a randomized selection of not-yet-scanned lines.
 9. Method according to claim 3, further including a second set of lines also including a first and a last line, and the steps of selecting, continuing selecting and scanning are performed on the second set of lines.
 10. Method according to claim 9, comprising the step of changing the scanning direction between at least some sequentially selected lines within the sets.
 11. Method according to claim 9, wherein the non-consecutive selection criterion comprises selecting a line in between the first and the last line of the first set and thereafter alternately selecting a lower order line and a higher order line relative to the first selected line until all lines of the first set have been scanned and selecting a line in between the first and the last line of the second set and thereafter alternately selecting a lower order line and a higher order line relative to the first selected line of the second set until all lines of the second set of lines have been scanned.
 12. Method according to claim 11, wherein a lower order line in the first set is selected simultaneously with a higher order line in the second set and a higher order line in the first set is selected simultaneously with a lower order line in the second set.
 13. Method according to claim 9, wherein the non-consecutive selection criterion comprises separate randomized selection of not-yet-scanned lines within each set.
 14. Method according to claim 1, comprising selecting a first selection criterion for a first frame, using said criterion for the first frame, selecting a second selection criterion for a second frame and using said second criterion for the second frame so that line selection and scanning is varied between frames.
 15. Display control (1) comprising: a line driving unit and a control unit, wherein the control unit is arranged to vary selection of lines to be scanned and to control the line driving unit to scan the selected lines so as to reduce tracking by a human eye of energy variations caused by scanning.
 16. Device according to claim 15, wherein the control unit is arranged to select lines to be scanned during a frame of the display according to a non-consecutive selection criterion and to control the line driving unit to scan the selected lines of the display until at least a set of all the lines of the display has been scanned during the frame, and wherein the non-consecutive selection criterion provides at least two different step sizes to be used when selecting lines within the set.
 17. Device according to claim 15, wherein the control unit is arranged to choose a first non-consecutive selection criterion, use said first criterion during at least a first frame, choose a second non-consecutive selection criterion and use said second criterion during at least a second frame so that line selection and scanning is varied between frames.
 18. An electronic device including a display and a display control device, wherein the display control device comprises: a line driving unit and a control unit, wherein the control unit is arranged to vary selection of lines to be scanned and to control the line driving unit to scan the selected lines so as to reduce tracking by a human eye of energy variations caused by scanning. 